The development of smaller circuits having improved specifications has been investigated to a certain extent. In some applications, it is found useful to use photonic chips, i.e. devices which involve the use of light directly on a chip in a manner analogous to the use of electricity in electronic chips.
These photonic chips are generally configured to propagate and process light from a photonic device to another via optical waveguides present on the photonic chip, conveniently referred to as chip waveguides. Typically, the incoming light is received from an external optical waveguide such as an optical fiber via direct coupling into one of the chip waveguides. Such direct coupling of the incoming light into the chip waveguides of the photonic chip can be challenging since these chip waveguides typically have cross-sectional areas much smaller than that of the external optical waveguide.
Vertical grating couplers can sometimes be used in vertical coupling configurations to increase the coupling efficiency between the external optical waveguide and a chip waveguide of the photonic chip. FIG. 1 shows an example of a vertical grating coupler 10 disposed on a photonic chip 12 and coupled to a chip waveguide 16 via a tapered waveguide 18. The vertical grating coupler 10 is specifically designed to receive light 20 emitted from an external optical waveguide 22 at an angle θ (e.g., smaller than 20°) close to normal incidence. As best seen in FIG. 1A, when using the vertical grating coupler 10, the resulting photonic chip 12 is rather bulky because the external optical waveguide 22 has to be maintained upright and tilted relative to the photonic chip 12 for efficient coupling.
Although existing coupling techniques have been found to be satisfactory up to a certain extent, there remains room for improvement.